JP5737276B2 - Cylinder air-fuel ratio variation abnormality detection device - Google Patents

Description

  The present invention relates to an inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine, and more particularly to detect an abnormality (imbalance abnormality) in which the air-fuel ratio of a certain cylinder is relatively large with respect to the air-fuel ratio of the remaining cylinders. Relating to the device.

  In general, in an internal combustion engine equipped with an exhaust gas purification system using a catalyst, a mixture ratio of air and fuel in an air-fuel mixture burned in the internal combustion engine, that is, an air-fuel ratio, is used to efficiently remove harmful components in exhaust gas with a catalyst. Control is essential. In order to perform such air-fuel ratio control, an air-fuel ratio sensor is provided in the exhaust passage of the internal combustion engine, and feedback control is performed so that the air-fuel ratio detected thereby coincides with a predetermined target air-fuel ratio.

  On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is normally performed using the same control amount for all cylinders. Therefore, even if air-fuel ratio control is executed, the actual air-fuel ratio may vary between cylinders. If the degree of variation is small at this time, it can be absorbed by air-fuel ratio feedback control, and harmful components in the exhaust gas can be purified by the catalyst, so that exhaust emissions are not affected and there is no particular problem.

  However, for example, if the fuel injection system of some cylinders breaks down and the air-fuel ratio between the cylinders varies greatly, exhaust emission deteriorates, causing a problem. It is desirable to detect such a large air-fuel ratio variation that deteriorates the exhaust emission as an abnormality. In particular, in the case of an internal combustion engine for an automobile, it is required to detect an abnormality in the air-fuel ratio variation between cylinders in an on-board state in order to prevent the vehicle from traveling with deteriorated exhaust emission.

JP 2011-47332 A

  When detecting an air-fuel ratio variation abnormality between cylinders, it is conceivable to calculate a parameter correlated with the output fluctuation degree of the air-fuel ratio sensor and compare the calculated parameter with a predetermined determination value to detect the variation abnormality.

  When using the output of the air-fuel ratio sensor, it is advantageous to improve the detection accuracy to consider the output characteristics (gain, response, etc.) of the actually installed air-fuel ratio sensor. For this reason, in Patent Document 1, it is determined in advance whether or not the output characteristic of the air-fuel ratio sensor is appropriate. If it is determined that the output characteristic is not appropriate, imbalance determination, that is, variation abnormality detection is prohibited.

  When considering the output characteristics of the actually installed air-fuel ratio sensor, as disclosed in Patent Document 1, the air-fuel ratio variation state is forcibly generated and the air-fuel ratio sensor output at this time is used. Can be considered.

  However, according to the research results of the present inventors, it has been found that it is not sufficient to simply use the air-fuel ratio sensor output when the air-fuel ratio variation state is forcibly generated. That is, it has been found that it is difficult to accurately detect the variation abnormality unless the air-fuel ratio sensor output before forcibly generating the air-fuel ratio variation state is also used.

  Accordingly, the present invention has been made in view of the above circumstances, and its object is to use the air-fuel ratio sensor output before forcibly generating the air-fuel ratio variation state, and to improve the detection accuracy of variation abnormality detection. An object of the present invention is to provide an inter-cylinder air-fuel ratio variation abnormality detection device.

According to one aspect of the invention,
A first parameter that correlates with an output fluctuation degree of an air-fuel ratio sensor installed in an exhaust passage common to a plurality of cylinders is calculated, and the calculated first parameter is compared with a predetermined determination value to vary the air-fuel ratio between cylinders. A cylinder-to-cylinder air-fuel ratio variation abnormality detection device for detecting an abnormality in a multi-cylinder internal combustion engine,
(A) calculating the first parameter;
(B) determining a possible range of the second parameter representing the degree of variation in air-fuel ratio between cylinders based on the calculated first parameter;
(C) forcibly changing the air-fuel ratio of a predetermined cylinder and calculating the first parameter;
(D) calculating a difference between the first parameters before and after the forced change;
(E) determining a first characteristic representing a relationship between the second parameter and the difference based on a possible range of the second parameter and the difference;
(F) correcting one of the determination value and the first parameter calculated in the step (A) based on the determined slope of the first characteristic;
An inter-cylinder air-fuel ratio variation abnormality detection device is provided that is configured to execute the above.

Preferably, in the step (E), the inter-cylinder air-fuel ratio variation abnormality detecting device
Using the first characteristic predetermined for the tolerance upper limit product and the tolerance lower limit product of the air-fuel ratio sensor, the difference calculated in the step (D) and the range that the second parameter can take The first characteristic passing through the intersection with the predetermined value is obtained by interpolation from the first characteristic of the tolerance upper limit product and the tolerance lower limit product, and the obtained first characteristic is determined as the first characteristic to be determined.

  Preferably, the predetermined value in the range that the second parameter can take is a value that minimizes the degree of variation in the range that the second parameter can take.

  Preferably, the first characteristic predetermined for the tolerance upper limit product and the tolerance lower limit product of the air-fuel ratio sensor is a property such that the difference increases as the second parameter changes to a larger variation degree side. .

  Preferably, the slope of the first characteristic predetermined for the tolerance upper limit product is larger than the slope of the first characteristic predetermined for the tolerance lower limit product.

Preferably, in the step (B), the inter-cylinder air-fuel ratio variation abnormality detecting device
The first parameter calculated in the step (A) using a second characteristic representing a relationship between the first and second parameters, which is predetermined for a tolerance upper limit product and a tolerance lower limit product of the air-fuel ratio sensor. And the range of the second parameter between the intersections of the tolerance upper limit product and the tolerance lower limit product with the second characteristic are determined as possible ranges of the second parameter.

  Preferably, the steps (A) and (C) include a step of normalizing the first parameter in accordance with an operating state of the internal combustion engine.

  According to the present invention, it is possible to improve the detection accuracy of variation abnormality detection by using an air-fuel ratio sensor output before forcibly generating an air-fuel ratio variation state.

1 is a schematic view of an internal combustion engine according to an embodiment of the present invention. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a graph which shows the fluctuation | variation of the exhaust air fuel ratio according to the air-fuel ratio variation degree between cylinders. FIG. 4 is an enlarged view corresponding to a U portion in FIG. 3. It is a table | surface for comparing the imbalance rate in various cases. It is a graph which shows the relationship between an imbalance rate and an output fluctuation parameter. It is a graph which shows the difference of the output fluctuation parameter before and behind forced imbalance execution in an ideal state. It is a graph which shows the difference of the output fluctuation parameter before and after the forced imbalance execution in practice. It is a graph for demonstrating the determination method of the range which an imbalance rate can take. It is a graph for demonstrating the determination method of the characteristic showing the relationship between an imbalance rate and a front-back difference. It is a graph for demonstrating the method of calculating | requiring the value of the output fluctuation parameter after forced imbalance execution, and by extension, a characteristic line about tolerance upper and lower limit goods. It is a figure which shows the correction map for correct | amending a determination value. It is a figure which shows the correction map for correct | amending an output fluctuation parameter. It is a graph for demonstrating the effect of this embodiment. It is a flowchart of an output fluctuation parameter calculation process. It is a flowchart of a variation abnormality detection process. It is a flowchart of the output fluctuation parameter calculation process in a modification. It is a figure which shows a rotation speed normalization map. It is a figure which shows a load normalization map.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic view of an internal combustion engine according to the present embodiment. As shown in the figure, an internal combustion engine (engine) 1 is powered by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston in the combustion chamber 3. Is generated. The internal combustion engine 1 of the present embodiment is a multi-cylinder internal combustion engine mounted on an automobile, more specifically, an in-line 4-cylinder spark ignition internal combustion engine. The internal combustion engine 1 includes # 1 to # 4 cylinders. However, the number of cylinders, the type, etc. are not particularly limited.

  Although not shown, the cylinder head of the internal combustion engine 1 is provided with an intake valve for opening and closing the intake port and an exhaust valve for opening and closing the exhaust port for each cylinder, and each intake valve and each exhaust valve is provided by a camshaft. Can be opened and closed. A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

  The intake port of each cylinder is connected to a surge tank 8 which is an intake air collecting chamber via a branch pipe 4 for each cylinder. An intake pipe 13 is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. In the intake pipe 13, an air flow meter 5 (intake air amount detection device) for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in order from the upstream side. An intake passage is formed by the intake port, the branch pipe 4, the surge tank 8 and the intake pipe 13.

  An injector (fuel injection valve) 12 for injecting fuel into an intake passage, particularly an intake port, is provided for each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture. The air-fuel mixture is sucked into the combustion chamber 3 when the intake valve is opened, compressed by the piston, and ignited and burned by the spark plug 7. The injector may inject fuel directly into the combustion chamber 3.

  On the other hand, the exhaust port of each cylinder is connected to the exhaust manifold 14. The exhaust manifold 14 includes a branch pipe 14a for each cylinder forming an upstream portion thereof and an exhaust collecting portion 14b forming a downstream portion thereof. An exhaust pipe 6 is connected to the downstream side of the exhaust collecting portion 14b. An exhaust passage is formed by the exhaust port, the exhaust manifold 14 and the exhaust pipe 6.

  Further, the exhaust passage downstream from the exhaust collecting portion 14b of the exhaust manifold 14 forms an exhaust passage common to the # 1 to # 4 cylinders that are a plurality of cylinders.

A catalyst composed of a three-way catalyst, that is, an upstream catalyst 11 and a downstream catalyst 19 are attached in series to the upstream side and the downstream side of the exhaust pipe 6, respectively. These catalysts 11 and 19 have an oxygen storage capacity (O ₂ storage capacity). That is, the catalysts 11 and 19 store excess oxygen in the exhaust gas and reduce NOx when the air-fuel ratio of the exhaust gas is larger (lean) than stoichiometric (theoretical air-fuel ratio, for example, A / F = 14.5). To do. Further, the catalysts 11 and 19 release the stored oxygen when the air-fuel ratio of the exhaust gas is smaller (rich) than the stoichiometric, and oxidize HC and CO in the exhaust gas.

  First and second air-fuel ratio sensors for detecting the air-fuel ratio of the exhaust gas, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 are installed on the upstream side and the downstream side of the upstream catalyst 11, respectively. The pre-catalyst sensor 17 and the post-catalyst sensor 18 are installed at positions immediately before and immediately after the upstream catalyst 11, and detect the air-fuel ratio based on the oxygen concentration in the exhaust gas. In this way, the single pre-catalyst sensor 17 is installed at the exhaust merging portion on the upstream side of the upstream catalyst 11. In this embodiment, the pre-catalyst sensor 17 corresponds to the “air-fuel ratio sensor” referred to in the present invention.

  The spark plug 7, the throttle valve 10, the injector 12, and the like described above are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as a control device or a control unit. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 16 that detects the crank angle of the internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. The opening sensor 15 and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the throttle valve 10, the injector 12, and the like according to various programs stored in the ROM so as to obtain a desired engine output based on detection values of various sensors, etc. The fuel injection amount, fuel injection timing, throttle opening, etc. are controlled.

  The throttle valve 10 is provided with a throttle opening sensor (not shown), and a signal from the throttle opening sensor is sent to the ECU 20. The ECU 20 normally feedback-controls the opening of the throttle valve 10 (throttle opening) to a target throttle opening determined according to the accelerator opening.

  Based on the signal from the air flow meter 5, the ECU 20 detects the intake air amount that is the amount of intake air per unit time, that is, the intake flow rate. The ECU 20 detects the load of the engine 1 based on at least one of the detected accelerator opening, throttle opening, and intake air amount.

  The ECU 20 detects the crank angle itself and the rotational speed of the engine 1 based on the crank pulse signal from the crank angle sensor 16. Here, “the number of rotations” means the number of rotations per unit time and is synonymous with the rotation speed. In the present embodiment, it means rpm per minute.

  The pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, and can continuously detect an air-fuel ratio over a relatively wide range. FIG. 2 shows the output characteristics of the pre-catalyst sensor 17. As shown in the figure, the pre-catalyst sensor 17 outputs a voltage signal Vf having a magnitude proportional to the exhaust air-fuel ratio. The output voltage when the exhaust air-fuel ratio is stoichiometric is Vreff (for example, about 3.3 V).

On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output value changes suddenly with the stoichiometric boundary. FIG. 2 shows the output characteristics of the post-catalyst sensor 18. As shown in the figure, the output voltage when the exhaust air-fuel ratio is stoichiometric, that is, the stoichiometric equivalent value is Vrefr (for example, 0.45 V). The output voltage of the post-catalyst sensor 18 changes within a predetermined range (for example, 0 to 1 V). When the exhaust air-fuel ratio is leaner than stoichiometric, the output voltage of the post-catalyst sensor becomes lower than the stoichiometric equivalent value Vrefr. When the exhaust air-fuel ratio is richer than stoichiometric, the output voltage of the post-catalyst sensor becomes higher than the stoichiometric equivalent value Vrefr.

  The upstream catalyst 11 and the downstream catalyst 19 simultaneously purify NOx, HC and CO, which are harmful components in the exhaust gas, when the air-fuel ratio A / F of the exhaust gas flowing into each of them is close to the stoichiometric range. The air-fuel ratio width (window) that can simultaneously purify these three with high efficiency is relatively narrow.

  Therefore, during normal operation, the air-fuel ratio feedback control is executed by the ECU 20 so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11 is controlled near the stoichiometric range. The air-fuel ratio feedback control is performed by a main air-fuel ratio control (main air-fuel ratio feedback control) that matches the exhaust air-fuel ratio detected by the pre-catalyst sensor 17 with a predetermined target air-fuel ratio, and a post-catalyst sensor 18. It consists of auxiliary air-fuel ratio control (auxiliary air-fuel ratio feedback control) that makes the detected exhaust air-fuel ratio coincide with stoichiometric.

  Note that air-fuel ratio feedback control in which the target air-fuel ratio is stoichiometric is called stoichiometric control. A stoichiometric air / fuel ratio is established.

  For example, a failure may occur in a part of all cylinders, particularly one cylinder, and an air-fuel ratio variation (imbalance) may occur between the cylinders. For example, the injector 12 of the # 1 cylinder fails, the fuel injection amount of the # 1 cylinder becomes larger than the fuel injection amount of the remaining # 2 to # 4 cylinder, and the air-fuel ratio of the # 1 cylinder is # 2 to # 4 cylinder This is the case when the air-fuel ratio is larger than the air-fuel ratio and shifts to the rich side. Even at this time, if a relatively large correction amount is given by the aforementioned stoichiometric control, the air-fuel ratio of the total gas supplied to the pre-catalyst sensor 17, that is, the average value of the air-fuel ratio of each cylinder may be controlled stoichiometrically. However, looking at each cylinder, # 1 cylinder is richer than stoichiometric, and # 2, # 3 and # 4 cylinders are slightly leaner than stoichiometric. It is clear. In view of this, the present embodiment is equipped with a device that detects such a variation in air-fuel ratio between cylinders.

  Hereinafter, an aspect of variation abnormality detection in the present embodiment will be described.

  As schematically shown in FIG. 3, the exhaust air-fuel ratio periodically fluctuates every engine cycle (= 720 ° CA). However, when a variation in the air-fuel ratio between cylinders occurs, the fluctuation of the exhaust air-fuel ratio within one engine cycle occurs. Becomes larger. The air-fuel ratio diagrams a, b, and c in (B) are not varied, and the pre-catalyst in the case of a rich shift at an imbalance rate of only one cylinder + 20% and a rich shift at an imbalance rate of only one cylinder + 50% An air-fuel ratio A / F detected by the sensor 17 is shown. As can be seen, the greater the degree of variation, the greater the amplitude of the air-fuel ratio fluctuation.

  Here, the imbalance rate is one parameter (second parameter) representing the degree of variation in the air-fuel ratio between cylinders. In other words, the imbalance rate is the ratio of the air-fuel ratio of the cylinder (imbalance cylinder) causing the air-fuel ratio deviation when only one cylinder among all the cylinders is causing the air-fuel ratio deviation. Is a value indicating whether or not there is a deviation from the air-fuel ratio of the cylinder (balance cylinder) that has not caused the air-fuel ratio deviation. In the case of this embodiment, the imbalance rate B is expressed by the following equation. The farther the imbalance rate B is from 1, the greater the air-fuel ratio deviation of the imbalance cylinder with respect to the balance cylinder, and the greater the air-fuel ratio variation.

  A / Fb is the air-fuel ratio of the balance cylinder, and A / Fib is the air-fuel ratio of the imbalance cylinder. For convenience, the imbalance rate may be displayed as a percentage. In this case, the imbalance rate B (%) is expressed by the following equation. As the absolute value of the imbalance rate B (%) increases, the air-fuel ratio deviation of the imbalance cylinder with respect to the balance cylinder increases, and the degree of air-fuel ratio variation increases.

  As understood from FIG. 3, the output fluctuation of the pre-catalyst sensor 17 increases as the absolute value of the imbalance ratio B (%) increases, that is, as the air-fuel ratio variation degree increases.

  Therefore, using this characteristic, in the present embodiment, the output fluctuation parameter X that is a parameter (first parameter) correlated with the output fluctuation degree of the pre-catalyst sensor 17 is calculated or detected, and the calculated output fluctuation parameter X is used as the calculated output fluctuation parameter X. Based on this, a variation abnormality is detected.

  A method for calculating the output fluctuation parameter X will be described below. FIG. 4 is an enlarged view corresponding to the U portion of FIG. 3, and particularly shows a fluctuation of the sensor output before the catalyst within one engine cycle. As the pre-catalyst sensor output, a value obtained by converting the output voltage Vf of the pre-catalyst sensor 17 into an air-fuel ratio A / F is used. However, the output voltage Vf of the pre-catalyst sensor 17 can be directly used.

As shown in FIG. 4B, the ECU 20 acquires the value of the pre-catalyst sensor output A / F for each predetermined sample period τ within one engine cycle. The value A / F _n obtained at the timing of this (n), (also referred to as output difference or sensor output difference) the difference between the value A / F _n-1 obtained at the timing of the previous (n-1) ΔA / F _{n is obtained} by the following equation. This output difference ΔA / F _n can be restated as a differential value of the pre-catalyst sensor output at the current timing.

Most simply, this output difference ΔA / F _n itself represents the magnitude of fluctuation in the sensor output before the catalyst. Therefore it is possible to output fluctuation parameter absolute value of the output difference .DELTA.A / F _n at the predetermined first timing. However, in this embodiment, in order to improve accuracy, an average value of a plurality of output differences ΔA / F _{n is used} as an output fluctuation parameter. In this embodiment, the output fluctuation parameter X is calculated by averaging the output difference ΔA / F _n for M engine cycles (for example, M = 50). The output fluctuation parameter X increases as the fluctuation degree of the pre-catalyst sensor output increases.

However, since the output difference ΔA / F _n has a positive case and a negative case, the calculation is performed by distinguishing them in the present embodiment. Details of the calculation method will be described later. However, it is also possible to calculate without making this distinction.

  Note that any value that correlates with the degree of fluctuation of the pre-catalyst sensor output can be used as the output fluctuation parameter. For example, output fluctuation based on the difference between the maximum peak and minimum peak of the sensor output before the catalyst within one engine cycle (so-called peak to peak), or the absolute value of the maximum peak or minimum peak of the second derivative. Parameters can also be calculated. This is because the difference between the maximum peak and the minimum peak of the pre-catalyst sensor output increases as the degree of fluctuation of the pre-catalyst sensor output increases, and the absolute value of the maximum peak or minimum peak of the second-order differential value also increases.

  Next, the calculated output fluctuation parameter X is compared with a predetermined determination value α to determine whether or not there is a variation abnormality. For example, if the calculated output fluctuation parameter X is greater than or equal to the determination value α, it is determined that there is a variation abnormality (abnormal), and if the calculated output fluctuation parameter X is less than the determination value α, it is determined that there is no variation abnormality (normal). In general, the determination value α is set in consideration of an OBD (On-Board Diagnosis) regulation value related to exhaust emission.

  By the way, as described above, when detecting variation abnormality using the pre-catalyst sensor output, it is advantageous to improve the detection accuracy to consider the output characteristics of the actually installed pre-catalyst sensor. In this case, it is conceivable that the air-fuel ratio variation state is forcibly generated and the air-fuel ratio sensor output at this time is used.

  However, according to the research results of the present inventors, it is not sufficient to use the pre-catalyst sensor output when the air-fuel ratio variation state is forcibly generated, and the air-fuel ratio variation state is forcibly generated. It has been found that it is difficult to detect variation abnormality with high accuracy unless the pre-catalyst sensor output in the previous state, that is, the running state is also used. Hereinafter, this point will be described.

  FIG. 5 mainly compares the imbalance rate in the case where there is no air-fuel ratio variation (imbalance) in the running state and in the case before and after execution of the forced imbalance that generates the forced air-fuel ratio variation state. It is a table for. Here, the state of execution and the state before forced imbalance execution have the same meaning, and both mean the state where normal control is being executed, that is, the state where stoichiometric control is being executed. The state after the forced imbalance is executed is a state where the forced imbalance is further executed while the base stoichiometric control is being executed.

  Here, the values of the fuel amount and the air-fuel ratio shown in (A) to (D) are all values after the air-fuel ratio of the total gas converges to stoichiometric (14.5) as a result of stoichiometric control.

  FIG. 5A shows a case where there is no imbalance in the growing state and before the forced imbalance is executed. As can be seen from the figure, an air amount of 14.5 and a fuel amount of 1 are supplied to all the cylinders, and the air-fuel ratio is stoichiometric = 14.5. Therefore, the imbalance rate is 14.5 / 14.5 = 1.00 = 0%.

  FIG. 5B shows a case where there is no imbalance in the running state and after forced imbalance execution. The air amount is 14.5 for all cylinders, but the fuel amount is 1.15 for the # 1 cylinder only, 0.95 for the other cylinders, the air-fuel ratio is 12.61 only for the # 1 cylinder, and 15 for the other cylinders. .26. The air-fuel ratio of # 1 cylinder is forcibly shifted to the rich side with respect to the air-fuel ratio of the other cylinders. Therefore, the imbalance rate is 15.26 / 12.61 = 1.105 = 21.05%.

  Forced imbalance or forced imbalance control is executed so that such a state is realized. That is, at the start of execution of forced imbalance, the fuel injection amount of only one predetermined cylinder, here, # 1 cylinder is forcibly increased by a predetermined amount, and the air-fuel ratio of only # 1 cylinder is forcibly shifted to the rich side. . Thereafter, the fuel injection amounts of all the cylinders are uniformly reduced and corrected so that the air-fuel ratio of the total gas becomes stoichiometric by stoichiometric control, and finally converges to the state shown in the figure. In this final state, the difference in fuel amount compared to before the execution of forced imbalance is +0.15 for the # 1 cylinder and -0.05 (= -0.15 / 3) for all other cylinders. The average value of the fuel amount in each cylinder is 1, and the air-fuel ratio of the total gas is stoichiometric.

  Here, since the imbalance rate after execution of forced imbalance is 21.05% higher than that before execution of forced imbalance in a banking state without imbalance, Bf = 1.105 = 21.05. This means that a forced imbalance equivalent to the% imbalance rate is being carried out. This Bf is referred to as a forced imbalance amount.

  Next, FIG. 5C shows a case where there is an imbalance in the mature state and before the forced imbalance is executed. As can be seen from the figure, the fuel amount is 1 for all the cylinders, but there is a difference in the air amount. Only 13 for the # 1 cylinder and 15 for the other cylinders. Therefore, the air-fuel ratio is 13 only for the # 1 cylinder and 15 for the other cylinders. Therefore, the imbalance rate is 15/13 = 1.538 = 15.38%. As a result of stoichiometric control, the air-fuel ratio of the total gas is stoichiometric.

  Thus, only the # 1 cylinder is larger (0.15) richer than stoichiometric and the other cylinders are slightly (0.5) leaner than stoichiometric. This state may occur due to an air amount deviation (insufficiency) of the # 1 cylinder. For example, in the # 1 cylinder, the cylinder-by-cylinder intake passage (branch pipe 4, intake port) is clogged by deposits or the intake valve This can happen when a valve opening failure occurs.

  FIG. 5D shows the result after executing the forced imbalance of the forced imbalance amount Bf in the same manner as described above. That is, FIG. 5D shows a case where there is an imbalance in the running state and after the forced imbalance is executed.

  The amount of air is 13 for the # 1 cylinder and 15 for the other cylinders as before the forced imbalance execution. As for the amount of fuel, as a result of execution of forced imbalance and stoichiometric control, only the # 1 cylinder is 1.15, and the other cylinders are 0.95. Therefore, the air-fuel ratio is 11.30 only for the # 1 cylinder and 15.79 for the other cylinders. The imbalance rate is 15.79 / 11.30 = 1.3967 = 39.67%.

  What should be noted here is that the imbalance rate (39.67%) after the forced imbalance execution is changed to the imbalance rate (15.38%) before the forced imbalance execution (21.05%). It is not the value (36.43%) when adding. The actual imbalance rate (39.67%) after execution of forced imbalance is larger by an error of 3.29% than the added value (36.43%).

  As shown in FIG. 6, there is a linear and first-order proportional relationship or characteristic (second characteristic) between the imbalance rate B and the output fluctuation parameter X. However, this relationship changes according to the output characteristics of the pre-catalyst sensor 17 (hereinafter also simply referred to as sensor output characteristics). In the figure, LXH indicates the characteristic or characteristic line of the tolerance upper limit product of the pre-catalyst sensor 17, and LXL indicates the characteristic or characteristic line of the tolerance lower limit product of the pre-catalyst sensor 17. As is well known, the tolerance upper limit product means the one that generates the highest output within the tolerance range for the same input, and the tolerance lower limit product means the one that generates the lowest output within the tolerance range for the same input. The slope of the characteristic line LXLH of the tolerance upper limit product is larger than the slope of the characteristic line LXL of the tolerance lower limit product.

For example, when considering the characteristic line LXH of the tolerance upper limit product, when the imbalance rate is set to B ₂ by executing the forced imbalance of the forced imbalance amount Bf from the state where there is no imbalance in the running state (B = B ₁ ). The output fluctuation parameter X changes from X ₁ to X ₂ , and the amount of change is X ₂ −X ₁ . On the other hand, when the forced imbalance of the forced imbalance amount Bf is executed from the state where there is an imbalance in the running state (B = B ₃ ) and the imbalance rate is set to B ₄ , the output fluctuation parameter X is changed from X ₃ to X _4. The amount of change is X ₄ -X ₃ . Since the forced imbalance amount Bf and the slope of the characteristic line LXH are constant, the amount of change is the same. Such a hypothetical state is called an ideal state.

  However, as you can see from the previous example, it does not actually become an ideal state. This is because even if the same amount of forced imbalance is apparently or controlled from the state where there is an imbalance in the running state, the amount of forced imbalance is not actually the same as when there is no imbalance in the running state. .

Similarly, considering the characteristic line LXL of the tolerance lower limit product, the forced imbalance of the forced imbalance amount Bf is executed from the state where there is no imbalance (B = B ₁ ) in the marketed state, and the imbalance rate is set to B ₂ . In this case, the output fluctuation parameter X changes from X ₁ ′ to X ₂ ′, and the amount of change is X ₂ ′ −X ₁ ′. On the other hand, when the forced imbalance of the forced imbalance amount Bf is executed from the state where there is an imbalance in the running state (B = B ₃ ) and the imbalance rate is B ₄ , the output variation parameter X is changed from X ₃ ′ to X ₄ 'changes to, the amount of change is X _4' is -X ₃ '. In the ideal state, both changes should be equal, but in reality this is not the case. In other words, no matter how the sensor output characteristic changes between the upper and lower tolerance products, it does not actually become an ideal state.

If the ideal state is obtained, the characteristics shown in FIG. 7 can be obtained. Here, DX on the vertical axis represents the amount of change, that is, the difference in the output fluctuation parameter X before and after execution of forced imbalance. The aforementioned X ₂ −X ₁ etc. correspond to this difference DX. B on the horizontal axis represents the imbalance rate before the forced imbalance is executed, that is, in the running state.

  As can be seen from the figure, the characteristic or characteristic line LDXH ′ representing the relationship between the imbalance rate B and the tolerance upper limit product difference DXH ′ is parallel to the horizontal axis, and the tolerance upper limit product difference DXH ′ is the imbalance rate. Regardless of B, it is constant. Similarly, the characteristic or characteristic line LDXL ′ representing the relationship between the imbalance rate B and the tolerance lower limit product difference DXL ′ is also parallel to the horizontal axis, and the tolerance lower limit product difference DXL ′ is independent of the imbalance rate B. It is constant. However, the difference DXH ′ of the tolerance upper limit product is larger than the difference DXL ′ of the tolerance lower limit product.

  However, in practice, characteristics as shown in FIG. 7 cannot be obtained.

  On the other hand, the present inventors found the following from the results of FIG. That is, the imbalance rate (1.3967) after the forced imbalance execution shown in FIG. 5D is changed to the imbalance rate (1.1538) before the forced imbalance execution shown in FIG. It becomes equal to the value when the balance amount (1.2105) is multiplied (1.1538 × 1.2105 = 1.3967).

  As a result, a characteristic (first characteristic) as shown in FIG. 8 is obtained instead of the characteristic as shown in FIG. As can be seen from FIG. 8, the characteristic or characteristic line LDXH representing the relationship between the imbalance rate B and the tolerance DXH of the tolerance upper limit product is a linear and linear proportional characteristic, and the difference DXH increases as the imbalance rate B increases. Becomes larger. Similarly, the characteristic or characteristic line LDXL representing the relationship between the imbalance ratio B and the tolerance lower limit product difference DXL is also a linear and linear proportional characteristic, and the difference DXL increases as the imbalance ratio B increases. However, the slope of the characteristic line LDXLH of the tolerance upper limit product is larger than the slope of the characteristic line LDXL of the tolerance lower limit product. The slope of the characteristic line LDX changes according to the sensor output characteristics, and the slope increases as the sensor output characteristics move toward the tolerance upper limit product side.

  Therefore, in this embodiment, the characteristics as shown in FIG. 8 are used. This characteristic uses the output fluctuation parameter X before the forced imbalance execution, that is, the pre-catalyst sensor output. Therefore, this characteristic takes into account the actual air-fuel ratio variation degree before the execution of forced imbalance. For this reason, according to the present embodiment, it is possible to improve the detection accuracy of variation abnormality detection.

  Note that the technique of Patent Document 1 described above does not consider the air-fuel ratio variation state before the forced change of the fuel injection amount. More specifically, it is based on the premise that there is no air-fuel ratio variation before the forced change of the fuel injection amount. Therefore, contrary to this assumption, if the air-fuel ratio has already fluctuated before the forced change of the fuel injection amount, it may not be possible to accurately determine whether the output characteristics of the air-fuel ratio sensor are appropriate. In spite of inappropriate output characteristics of the air-fuel ratio sensor, the variation abnormality detection is executed, which may lead to erroneous detection. However, according to the present embodiment, these drawbacks can be overcome.

  Hereinafter, an outline of variation abnormality detection in the present embodiment will be described. The detection is executed mainly by the ECU 20 executing the following steps.

(A) Step of calculating the output fluctuation parameter X1 In this step, the value X1 of the output fluctuation parameter before the forced imbalance execution, that is, in the running state is calculated or detected by the method described above.

(B) Step of determining the range DB1 that the imbalance rate can take based on the calculated output variation parameter X1 In this step, the relationship between the imbalance rate B and the output variation parameter X as shown in FIG. 6 is used. To do. That is, as shown in FIG. 9, the characteristics or characteristic lines LXH and LXL representing the relationship between the imbalance rate B and the output fluctuation parameter X are first obtained in advance for the tolerance upper limit product and the tolerance lower limit product of the pre-catalyst sensor 17. deep. These characteristics or characteristic lines LXH and LXL are obtained by a fitting operation through an actual machine test using an actual tolerance upper limit product and a tolerance lower limit product. The obtained characteristics are stored in the ECU 20 in advance.

  Then, intersections between the output fluctuation parameter X1 actually calculated in step (A) and the characteristic lines LXH and LXL of the tolerance upper limit product and the tolerance lower limit product are obtained. As shown in FIG. 9, the coordinates of the two intersections are (B1, X1) and (B2, X1). The range B1 ≦ B ≦ B2 of the imbalance rate between these intersections is determined as the range DB1 that the imbalance rate can take. That is, the range of the imbalance rate B is limited based on the actually calculated output fluctuation parameter X1.

  In the present embodiment, the pre-catalyst sensor 17 that is actually installed is normal and is based on the premise that it has any output characteristic between the tolerance upper limit product and the tolerance lower limit product. Therefore, when the output fluctuation parameter of X1 is actually obtained, the imbalance rate B at that time should be any value in the range of B1 ≦ B ≦ B2. Therefore, it is determined as a range DB1 that can take this range.

  This possible range DB1 is a range according to the actual air-fuel ratio variation degree in the running state, and becomes narrow when the actual air-fuel ratio variation degree is small, and widens when the actual air-fuel ratio variation degree is large. In any case, the range DB1 represents a range of variation in output characteristics of the pre-catalyst sensor 17 that is actually installed.

(C) A step of executing a forced imbalance forcibly changing the air-fuel ratio of a predetermined one cylinder and calculating an output fluctuation parameter X2. In this step, the aforementioned forced imbalance is executed, and after the forced imbalance is executed, A value X2 of the output fluctuation parameter is calculated. Note that “after forced imbalance is executed” means that forced imbalance is being executed. When the forced imbalance is executed, the fuel injection amount of a predetermined cylinder (this is called a forced imbalance cylinder) is forcibly or actively increased by a predetermined forced imbalance amount Bf. The forced imbalance amount Bf is, for example, equivalent to an imbalance rate of 1.1 = 10%.

  Here, the forced imbalance cylinder is a cylinder that has the highest probability that a rich shift imbalance has occurred or has occurred in the normal state. In other words, the forced imbalance is a control that further emphasizes the air-fuel ratio shift state of one cylinder that has the highest probability or the occurrence of the air-fuel ratio shift in the running state. Therefore, this embodiment also has a function of selecting or specifying such one cylinder as a forced imbalance cylinder.

  As shown in FIG. 4, ignition and combustion are performed in the order of the cylinders # 1, # 3, # 4, and # 2 in one engine cycle, and the pre-catalyst sensor output A / F according to the exhaust air-fuel ratio of each cylinder. Changes. In the figure, TDC means compression top dead center. The illustrated example shows a case where the rich shift imbalance occurs in the # 4 cylinder in the running state. As shown in the figure, the pre-catalyst sensor output A / F decreases to the rich side relatively rapidly when the pre-catalyst sensor 17 receives the exhaust gas from the # 4 cylinder, and otherwise the pre-catalyst sensor output A / F. Has increased relatively slowly to the lean side.

Thus in the present embodiment performs the correspondence between the pre-catalyst sensor output A / F and the output difference .DELTA.A / F _n and the respective cylinders, with an average value of the output difference .DELTA.A / F _n for each cylinder, the average value The cylinder that is the largest on the negative side is identified as the forced imbalance cylinder.

  Alternatively or additionally, one cylinder that has the highest probability of occurrence of lean deviation imbalance or that has the highest probability is defined as a forced imbalance cylinder, and the fuel injection amount is forced to the forced imbalance cylinder. Alternatively, the forced imbalance may be executed by reducing the amount by a predetermined amount. Also, other methods including a known method can be used as the method for specifying the forced imbalance cylinder.

(D) Step of calculating output fluctuation parameter difference DX1 before and after execution of forced imbalance In this step, output fluctuation parameter value X1 calculated in step (A) and output fluctuation parameter calculated in step (C) A difference DX1 = X2−X1 from the value X2 is calculated.

(E) A step of determining a characteristic representing the relationship between the imbalance rate B and the difference DX1 based on the range DB1 that can be taken by the imbalance rate and the difference DX1. In this step, the imbalance rate B as shown in FIG. And the difference between the output fluctuation parameters before and after the forced imbalance execution (referred to as the difference between before and after) DX. That is, as shown in FIG. 10, the characteristics or characteristic lines LDXH and LDXL representing the relationship between the imbalance rate B and the front-back difference DX are first determined in advance for the tolerance upper limit product and the tolerance lower limit product of the pre-catalyst sensor 17. . These characteristics or characteristic lines LDXH and LDXL are obtained through an actual machine test or the like by actually performing forced imbalance using actual tolerance upper limit products and tolerance lower limit products. The obtained characteristics are stored in the ECU 20 in advance. It should be noted that it is also possible to obtain the value of the output fluctuation parameter after the forced imbalance is executed and thus the characteristic lines LDXH and LDXL by calculation without actually executing the forced imbalance at the adaptation stage. This method will be described later.

  Then, the characteristic or characteristic line to be determined in this step is determined based on the difference DX1 actually calculated in step (D) and the range DB1 that the imbalance rate B determined in step (B) can take. decide. Specifically, the characteristic line or characteristic line passing through the intersection of a predetermined value in the range DB1 that the imbalance ratio B can take and the actually calculated front-rear difference DX1 is the characteristic line of the tolerance upper limit product and the tolerance lower limit product. Obtained by interpolation from LDXH and LDXL. Then, the obtained characteristic or characteristic line is determined as the characteristic or characteristic line to be determined in this step.

  In the present embodiment, the predetermined value in the range DB1 that the imbalance rate B can take is a value that minimizes the air-fuel ratio variation in the possible range, that is, the minimum imbalance rate B1. Then, a characteristic or characteristic line LDXB1 passing through the intersection of the imbalance rate B1 and the longitudinal difference DX1 is obtained by interpolation from the characteristic lines LDXH and LDXL of the tolerance upper limit product and tolerance lower limit product.

  As shown in FIG. 10, the coordinates of the intersection are (B1, DX1). The virtual intersection point of the characteristic lines LDXH and LDXL of the tolerance upper limit product and tolerance lower limit product is P. A straight line passing through the virtual intersection P and passing through the intersection (B1, DX1) is obtained by calculation. The obtained straight line is determined as the characteristic line LDXB1. This characteristic line LDXB1 corresponds to the output characteristic of the pre-catalyst sensor 17 that is actually installed.

  For reference, the characteristic or characteristic line LDXB2 when the predetermined value in the range DB1 that the imbalance rate B can take is a value that maximizes the degree of variation in the air-fuel ratio, that is, the maximum imbalance rate B2, is shown in FIG. It is written together. The characteristic or characteristic line to be determined in this step can be an arbitrary one passing through the virtual intersection P and between the characteristic line LDXB1 and the characteristic line LDXB2. However, the predetermined value is preferably the minimum imbalance rate B1. The reason will be explained later.

Here, a description will be given of a method for obtaining the value of the output fluctuation parameter after the execution of the forced imbalance by calculation, and thus the characteristic lines LDXH and LDXL, without actually executing the forced imbalance at the adapting stage. Here, known characteristic lines LXH and LXL as shown in FIG. 9 are used. For example, as shown in FIG. 11, attention is paid to a point (B ₁ , X ₁ ) on the characteristic line LXH of the tolerance upper limit product before execution of forced imbalance. If the forced imbalance is executed from this state, the imbalance rate B ₂ after the forced imbalance is obtained by B ₂ = B ₁ × Bf. As a result, the value X ₂ of the output fluctuation parameter on the characteristic line LXH corresponding to B ₂ is obtained. The value X ₂ of the output fluctuation parameter is associated with the imbalance rate B ₁ before the forced imbalance execution. Accordingly, the point (B ₁ , X ₂ ) is one point that represents the value X ₂ of the output fluctuation parameter after the forced imbalance execution corresponding to the imbalance rate B ₁ before the forced imbalance execution.

The same operation is performed for another point (B ₃ , X ₃ ) before the forced imbalance execution. Then, another point (B ₃ , X ₄ ) representing the output variation parameter value X ₄ after the forced imbalance execution corresponding to the imbalance rate B ₃ before the forced imbalance execution is obtained. If a straight line passing through the point (B ₁ , X ₂ ) and the point (B ₃ , X ₄ ) is obtained, this is a characteristic line LXH representing the relationship between the imbalance rate B and the output fluctuation parameter X after execution of forced imbalance. "

  The characteristic line LDXH as shown in FIG. 8 and FIG. 10 can be obtained by obtaining the front-back difference DX obtained by subtracting the value on the characteristic line LXH from the value on the characteristic line LXH ”for each imbalance rate B. it can.

  By performing the same operation on the characteristic line LXL of the tolerance lower limit product, a characteristic line LXL ″ (not shown) representing the relationship between the imbalance rate B and the output fluctuation parameter X after the forced imbalance execution is obtained. A characteristic line LDXL representing the relationship between the imbalance rate B and the front-back difference DX can be obtained.

(F) Step of correcting one of determination value α and output fluctuation parameter X based on the determined slope of characteristic LDXB1 First, a method of correcting determination value α will be described. As described above, the determination value α is a threshold value that is compared with the output fluctuation parameter X that is actually calculated to determine the presence or absence of variation abnormality.

  First, the slope S (LDXB1) of the characteristic line LDXB1 determined in step (E) is calculated or acquired. Note that the slope here means the ratio of the amount of change in the front-to-back difference DX to the amount of change in the imbalance rate X on the characteristic line, as shown in FIG. 10, which is exactly the slope of the characteristic line shown in the figure.

  Then, according to the correction map as shown in FIG. 12, a determination value α1 corresponding to the slope S (LDXB1) is obtained, and the reference determination value α0 is corrected as a result. The reference determination value α0 is a threshold that is determined in advance as the pre-catalyst sensor 17 that is actually installed is a tolerance upper limit product. Therefore, the reference determination value α0 corresponds to the slope S (LDXH) of the characteristic line LDXH of the tolerance upper limit product.

  As can be understood from FIG. 12, as the slope S of the characteristic line LDX becomes smaller than the slope S (LDXH) of the tolerance upper limit product, the determination value α also becomes smaller than the reference determination value α0. Therefore, the determination value α is corrected to a smaller value α1 commensurate with the actual sensor output characteristics with respect to the reference determination value α0 determined in advance on the assumption of the tolerance upper limit product.

  Next, a method for correcting the output fluctuation parameter X will be described. In this correction, a correction map as shown in FIG. 13 is used. From this correction map, a correction coefficient J1 corresponding to the slope S (LDXB1) is obtained. The output fluctuation parameter X is corrected by multiplying the output fluctuation parameter X by the correction coefficient J1. The reference value of the correction coefficient J is a value of 1 determined in advance as the pre-catalyst sensor 17 actually installed is a tolerance upper limit product. Therefore, the correction coefficient J = 1 corresponds to the slope S (LDXH) of the characteristic line LDXH of the tolerance upper limit product.

  As can be understood from FIG. 13, the correction coefficient J increases from the reference value = 1 as the slope S of the characteristic line LDX decreases from the slope S (LDXH) of the tolerance upper limit product. Therefore, the output fluctuation parameter X is corrected to a larger value that matches the actual sensor output characteristics.

  Similarly to this, when correcting the determination value α, a correction coefficient may be obtained and multiplied by the reference determination value α0. These correction methods can be other methods.

(G) A step of comparing the corrected determination value α (or output fluctuation parameter X) with the uncorrected output fluctuation parameter X (or determination value α) to determine the presence or absence of variation abnormality. As described above. Since the determination value α (or the output fluctuation parameter X) is corrected in accordance with the actual sensor output characteristics, variation abnormality detection can be performed with high accuracy.

  The above is the outline of the variation abnormality detection of the present embodiment. Below, the effect of this embodiment is additionally demonstrated.

  As described above, the range DB1 that the imbalance ratio can take is determined in step (B) (see FIG. 9), and in step (E), the characteristic line LDXB1 corresponding to the actual front-to-back difference DX1 is based on this range DB1. Is determined (see FIG. 10). In step (F), the corrected determination value α1 is determined based on the slope S (LDXB1) of the characteristic line (see FIG. 12).

  In the present embodiment, when the characteristic line LDXB1 is determined, a straight line passing through the intersection of the minimum imbalance rate B1 and the front-back difference DX1 in the range DB1 is determined as the characteristic line LDXB1. That is, the predetermined value in the range DB1 is set to the minimum imbalance rate B1.

  The reason why this is preferable is as follows. As shown in FIG. 9, the minimum imbalance rate B1 is a value corresponding to the characteristic line LXH of the tolerance upper limit product. The tolerance upper limit product is the one that maximizes the value of the output fluctuation parameter X with respect to the same imbalance rate, that is, the one that moves to the variation abnormality side. Thus, it is appropriate to determine the determination value α in correspondence with the tolerance upper limit product. This is because when the determination value α is determined in correspondence with the tolerance lower limit product, the corrected determination value α is determined based on the smaller slope of the characteristic line LDXB2 in FIG. 10, and the corrected determination value α is smaller. Because it becomes. At this time, if a tolerance upper limit product is actually attached, it may be erroneously determined that there is a variation abnormality although there is no variation abnormality originally. In order to suppress such erroneous determination, it is effective to determine the characteristic line LDXB1 based on the minimum imbalance rate B1.

  As shown in FIG. 10, when the characteristic line LDXB1 having the maximum inclination and the characteristic line LDXB2 having the minimum inclination corresponding to the actual front-to-back difference DX1 are known, the range in which the imbalance ratio can be taken is substantially further limited. it can.

As shown in FIG. 14, there is an imbalance ratio X range in which it is not desired to determine that there is a variation abnormality, regardless of what the pre-catalyst sensor is between the upper and lower tolerance products in the adaptation stage. In the present embodiment, this is, for example, less than 20%. From this point of view, B ₁ = 20% is set, and the value of the output fluctuation parameter X corresponding to B ₁ on the characteristic line LXH of the tolerance upper limit product is set as the reference determination value α0. In this case, the actual pre-catalyst sensor attached shifts to lower tolerance products side, up to a maximum imbalance rate B _2, thus can not be determined that there is variation abnormality. In the present embodiment, B _{2 is set} to 50%, for example. Eventually, the range where B <B ₁ can be accurately determined to be normal regardless of the sensor output characteristics, the range where B> B ₂ can be accurately determined as abnormal regardless of the sensor output characteristics, B ₁ ≦ B ≦ B The range DB of _{2 is} a range in which the determination of normality and abnormality differs according to sensor output characteristics, that is, a gray zone.

On the other hand, the characteristic line representing the relationship between the imbalance ratio X and the output fluctuation parameter X corresponding to both characteristic lines LDXB1 and LDXB2 based on the actual difference DX1 is between the characteristic lines LXH and LXL of the tolerance upper and lower limit products. LXB ₁ and LXB ₂ are sandwiched. Determination value α1 corrected based on LXB ₁ is determined. The range of the imbalance rate between the intersections of the corrected determination value α1 and the characteristic lines LXB ₁ and LXB ₂ is DB ′.

The range DB 'is B ₁ ≦ B ≦ B _2' is in the range of the minimum value is equal in B _1, the maximum value is adapted when the B ₂ smaller B ₂ '. In the present embodiment, B ₂ ′ is 30%, for example. Thus, the range DB ′ is reduced more than the range DB determined in the adaptation stage. This range DB ′ is a possible range of the imbalance rate X corresponding to the actually mounted pre-catalyst sensor. Therefore, the range that the imbalance rate X can take is further limited than that at the time of adaptation.

In this case, the range that can be accurately determined to be normal regardless of the sensor output characteristics is B <B _1, which is the same as that in the adaptation. However, the minimum value of the range that can be accurately determined as abnormal regardless of the sensor output characteristics is changed to a smaller B ₂ ′, so that it can be accurately determined as abnormal from the smaller imbalance rate X. The range in which the normal and abnormal determinations differ according to the sensor output characteristics, that is, the gray zone, is limited to a narrower range of B ₁ ≦ B ≦ B ₂ ′, and the imbalance rate X that may cause an erroneous determination. The range can be reduced. As a result, it is possible to increase the detection accuracy of variation abnormality detection.

  Next, more specific detection processing for variation abnormality detection according to the present embodiment will be described.

  First, calculation processing of the output fluctuation parameter X, which is basic processing of the present embodiment, will be described. The calculation process is performed by the ECU 20 repeatedly executing a routine as shown in FIG. 15 at predetermined calculation cycles.

First, in step S101, the pre-catalyst sensor output A / F _n at the current sample time or timing _n is acquired. Then, the sensor output difference ΔA / F _{n at the} current timing is calculated from the previous equation (2). Here, both the value of the pre-catalyst sensor output A / F _n, and the sensor output difference .DELTA.A / F _n is, two values is the discharge destination of the cylinder number of the exhaust gas led to associated, set the two values and cylinder number Is stored in the ECU 20. This is to specify the forced imbalance cylinder later.

Next, in step S102, it is determined whether or not the sensor output difference ΔA / F _{n at the} current timing is greater than zero.
If it is greater than zero, that is, if the sensor output difference (slope) ΔA / F _n at this time is a positive value and is a value when the sensor output before the catalyst is increased, in step S103, a positive sensor output at the current time is obtained. The difference ΔA / F _n is integrated, and the integrated value ΣΔA / F _{n +} is calculated from the following equation (3).

In step S104, the number of integrations C1 ₊ of positive value of the sensor output difference .DELTA.A / F _n is incremented by 1 (increment).

On the other hand, in step S102, if the sensor output difference ΔA / F _{n at the} current timing is less than or _equal to zero, that is, the sensor output difference (slope) ΔA / F _{n at the} current timing is zero or minus, If the value is the value at the time of change or decrease, the negative sensor output difference ΔA / F _n at the current timing is integrated in step S105, and the integrated value ΣΔA / F _n− is calculated from the following equation (4). The

In step S106, the number of integrations C1 minus sensor output difference ΔA / F _{n -} value is incremented by 1 (increment).

Next, in step S107, it is determined whether or not the crank angle θ at the current timing is 0 ° CA, which is a reference crank angle in one engine cycle (0 to 720 ° CA). This reference crank angle defines the timing for calculating the average value of the sensor output differences ΔA / F _n during one engine cycle. The reference crank angle can be set to a value other than 0 ° CA. In the present embodiment, the reference crank angle of 0 ° CA is equal to the compression top dead center of the # 1 cylinder (see FIG. 4).

If the crank angle θ is not 0 ° CA, the routine is terminated. On the other hand, if the crank angle θ is 0 ° CA, in step S108, with the average value of the sensor output difference .DELTA.A / F _n at the current one engine cycle at the end is calculated, this average value is accumulated. First, for the positive sensor output difference ΔA / F _n , the integrated value ΣΔA / F _{n +} of the positive sensor output difference is divided by the number of times of integration C1 ₊ to obtain an average value R _{m +} (= (ΣΔA / F _{n +} ) for each engine cycle. / C1 ₊ ) is calculated. And this average value R _{m +,} is added to the integrated value of the average value for each engine cycle, the mean value R _{m +} integrated value .SIGMA.R m ₊ is obtained. The integrated value ΣR _{m +} is calculated from the following equation (5).

Similarly, for the negative sensor output difference ΔA / F _n , the integrated value ΣΔA / F _n− of the negative sensor output difference is divided by the number of times of integration C1 ₋ to obtain an average value R _m− (= (ΣΔA / F _n− ) / C1 ₋ ) is calculated. Then the average value R _m-may be added to the integrated value of the average value for each engine cycle, the mean value R _m-integrated value .SIGMA.R _m-is obtained. The integrated value ΣR _m− is calculated from the following equation (6).

Next, in step S109, the number of times C2 ₊ and C2 ₋ of the plus average value R _{m +} and the minus average value R _m− for each engine cycle is incremented by 1 (incremented).

Thereafter, in step S110, the integrated value ΣΔA / F _{n +} of the positive sensor output difference and the integrated value ΣΔA / F _n− of the negative sensor output difference are cleared to zero. In step S111, positive sensor output difference between the number of integrations C1 ₊ and negative sensor output difference between the number of integrations C1 _- values of are cleared to zero.

Then, in step S112, accumulated number of positive average value for each engine cycle C2 ₊ reached more than ₊ a predetermined threshold value M, and number of integrations negative average value for each engine cycle C2 _- a predetermined threshold value M _- It is determined whether or not the above has been reached. In the present embodiment, for example, M ₊ = M ₋ = 50. If no, the routine is terminated.

On the other hand, If yes, the integration in step S113, M ₊ average value of the engine cycle and (ΣR _{m +)} / C2 + obtained by dividing an integrated value .SIGMA.R _{m +} accumulation number C2 _+, the integrated value .SIGMA.R _m- An average value (ΣR _m− ) / C2 ₋ during the M ₋ engine cycle divided by the number of times C2 ₋ is calculated. Then, an output fluctuation parameter X is calculated based on both average values.

  In the present embodiment, the average value of the absolute values of both average values is calculated as the output fluctuation parameter X. However, other values are possible. For example, the larger of the absolute values of both average values or the sum of the absolute values of both average values may be calculated as the output fluctuation parameter X. If the output fluctuation parameter X is calculated in this way, the routine is terminated.

  Next, the variation abnormality detection process of this embodiment will be described. The detection process is executed by the ECU 20 according to an algorithm as shown in the flowchart of FIG.

First, in step S201, it is determined whether or not a predetermined precondition suitable for executing variation abnormality detection is satisfied. For example, the precondition is satisfied when the following conditions are satisfied. Other examples are possible for each condition.
(1) The engine has been warmed up.
(2) The pre-catalyst sensor 17 and the post-catalyst sensor 18 are activated.
(3) The upstream catalyst 11 and the downstream catalyst 19 are activated.
(4) The engine is in steady operation.
(5) The engine speed Ne and the load KL are within a predetermined range.
(6) The stoichiometric control is in progress.

  If the precondition is not satisfied, the process waits. If the precondition is satisfied, the process proceeds to step S202. In step S202, the value of the output fluctuation parameter X before the forced imbalance execution is calculated. This calculation is performed by executing the routine shown in FIG.

  In step S203, as shown in FIG. 9, a range DB that the imbalance ratio can take is determined based on the calculated value of the output fluctuation parameter X.

In step S204, a forced imbalance cylinder is specified. At this time, the set data of the sensor output difference ΔA / F _n and the cylinder number acquired in step S101 of the routine of FIG. 15 is used.

For example, an average value (whether plus or minus) of the sensor output difference ΔA / F _n for each cylinder number is obtained, and the cylinder having the largest absolute value of this average value is specified as the forced imbalance cylinder. As understood from FIG. 4, the absolute value of the average value of the sensor output difference ΔA / F _n of the # 4 cylinder in which the rich shift imbalance occurs in the running state is larger than that of the other cylinders. Therefore, it is possible to specify the forced imbalance cylinder by this method.

Here, it is also determined whether the forced imbalance cylinder is in an improper state and has a rich deviation imbalance (or is it possible) or a lean deviation imbalance. At this time, if the average value of the sensor output difference ΔA / F _n of the forced imbalance cylinder is positive, it is specified as lean shift imbalance, and if it is negative, it is specified as rich shift imbalance.

  In step S205, forced imbalance is executed. That is, the fuel injection amount is increased or decreased by a predetermined amount with respect to the forced imbalance cylinder. At this time, if the forced imbalance cylinder is specified as rich shift imbalance in step S204, the fuel injection amount is increased so as to emphasize the rich shift state. Conversely, if the forced imbalance cylinder is specified as lean deviation imbalance in step S204, the fuel injection amount is reduced so as to emphasize the lean deviation state.

  In step S206, the value of the output fluctuation parameter X after execution of forced imbalance is calculated. This calculation is performed again by executing the routine shown in FIG.

  In step S207, the difference between the output fluctuation parameters X before and after the forced imbalance execution, that is, the front-back difference DX is calculated. This calculation is performed by subtracting the value of the output fluctuation parameter X calculated in step S202 from the value of the output fluctuation parameter X calculated in step S206.

  In step S208, as shown in FIG. 10, a characteristic line LDX (representing the relationship between the imbalance rate B and the front-to-back difference DX) based on the calculated front-to-back difference DX and the possible range DB of the imbalance rate. = LDXB1) is determined.

  In step S209, the slope S (= S (LDXB1)) of the characteristic line LDX (= LDXB1) is determined.

  In step S210, as shown in FIG. 12, a corrected determination value α (= α1) is determined based on the determined slope S (= S (LDXB1)).

  In step S211, the value of the output fluctuation parameter X before the forced imbalance execution calculated in step S202 is compared with the corrected determination value α determined in step S210.

  If the value of the output fluctuation parameter X is less than the determination value α, it is determined in step S212 that there is no variation abnormality, that is, normal.

  If the value of the output fluctuation parameter X is greater than or equal to the determination value α, it is determined in step S213 that there is a variation abnormality, that is, an abnormality. At this time, a warning device such as a check lamp is activated, the fact of the abnormality is notified to the user, and the user is prompted to repair.

  Next, a modification of this embodiment will be described.

  The actual engine operating state is different before and after the execution of forced imbalance, and there is a possibility that the calculation accuracy of the difference DX is reduced due to this. As a result, the variation abnormality detection accuracy may be reduced.

  Therefore, in this modified example, in order to suppress such a decrease in accuracy, normalization is performed so that the value of the output fluctuation parameter X calculated before and after the execution of the forced imbalance becomes a value when the engine operating condition is constant. Do.

  As a result, the operating conditions of the output fluctuation parameter X calculated before and after the execution of forced imbalance are substantially the same, and the calculation accuracy of the front-to-back difference DX and thus the variation abnormality detection accuracy can be improved.

  FIG. 17 shows a flowchart of output fluctuation parameter calculation processing according to a modification. The calculation process is almost the same as the calculation process shown in FIG. 15, and the description of the same steps is omitted only by changing the code to the 300s. The only difference is that step S108 is replaced with step S308A, and normalization processing is performed in step S308A.

In step S308A, the average values R _{m +} and R _m− for each engine cycle calculated in the current engine cycle are average engine operating states during the engine cycle, specifically, the detected engine speed Ne. And the engine load KL are normalized according to the average value. Then, the average values R _{m +} and R _m− after normalization are integrated.

For this normalization, a predetermined map as shown in FIGS. 18 and 19 is used. In the rotation speed normalization map shown in FIG. 18, when the engine rotation speed Ne is an idle rotation speed Nei that is a predetermined normalization rotation speed, the rotation speed normalization coefficient K1 is 1, and the rotation speed increases as the engine rotation speed Ne increases. The number normalization coefficient K1 increases. A rotation speed normalization coefficient K1 corresponding to the detected engine rotation speed Ne is obtained from the map, and the rotation speed normalization coefficient K1 is multiplied by the respective average values R _{m +} and R _m− . Thereby, normalization regarding the engine speed is performed.

In general, the sensor output difference ΔA / F tends to decrease as the engine speed Ne increases. Therefore, by performing normalization as described above, it is possible to normalize the average values R _{m +} and R _m− detected at an arbitrary rotational speed to the values at the idle rotational speed Nei.

Similarly, in the load normalization map shown in FIG. 19, when the engine load KL is an idle load KLi that is a predetermined normalization load, the load normalization coefficient K2 is 1, and the load normalization coefficient increases as the engine load KL increases. K2 becomes smaller. A load normalization coefficient K2 corresponding to the detected engine load KL is obtained from the map, and the load normalization coefficient K2 is multiplied by the respective average values R _{m +} and R _m− . This normalizes the engine load.

In general, the sensor output difference ΔA / F tends to increase as the engine load KL increases. Therefore, by performing normalization as described above, the average values R _{m +} and R _m− detected at an arbitrary load can be normalized so as to be the values at the time of the idle load KLi.

After all, (R _{m +} × K1 × K2) is calculated as a positive average value after normalization, and (R _m− × K1 × K2) is calculated as a negative average value after normalization, and these values are integrated. By executing the subsequent steps based on these integrated values, the normalized value of the output fluctuation parameter X can be obtained as a result.

  In steps S202 and S206 of FIG. 16, the normalized value of the output fluctuation parameter X is calculated. Therefore, the value of the longitudinal difference DX calculated using these values in step S207 is also set to a value under a certain operating condition. Will be aligned. Therefore, the calculation accuracy of the front-back difference DX can be improved, and the variation abnormality detection accuracy can be improved.

In this embodiment, normalization is performed by multiplying the normalization coefficients K1 and K2, but normalization may be performed by addition or the like. Further, normalization may be performed on each sensor output difference ΔA / F _n obtained in step S301, the value of the output fluctuation parameter X finally obtained in step S313, and the like.

  The preferred embodiment of the present invention has been described in detail above, but various other embodiments of the present invention are conceivable. For example, the above numerical values are merely examples, and various changes can be made. Further, in the above description, there is a place where only one of the rich shift imbalance and the lean shift imbalance is described. Let's go.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 Internal combustion engine
2 Injector 6 Exhaust pipe 11 Upstream catalyst 14 Exhaust manifold 17 Pre-catalyst sensor 18 Post-catalyst sensor 19 Downstream catalyst 20 Electronic control unit (ECU)
X Output fluctuation parameter α Determination value B Imbalance rate DB Possible range of imbalance rate DX Front-back difference LDX Characteristic S representing the relationship between imbalance rate and front-back difference Slope K1 Speed normalization coefficient K2 Load normalization coefficient

Claims (7)

A first parameter that correlates with an output fluctuation degree of an air-fuel ratio sensor installed in an exhaust passage common to a plurality of cylinders is calculated, and the calculated first parameter is compared with a predetermined determination value to vary the air-fuel ratio between cylinders. A cylinder-to-cylinder air-fuel ratio variation abnormality detection device for detecting an abnormality in a multi-cylinder internal combustion engine,
(A) calculating the first parameter;
(B) based on the calculated first parameter, the air-fuel ratio variation degree between the cylinders and determining a table to and possible range of the first parameter is different from a second parameter, the second parameter, A step which is a value indicating a deviation ratio of the air-fuel ratio of the cylinder causing the air-fuel ratio deviation to the air-fuel ratio of the cylinder not causing the air-fuel ratio deviation ;
(C) forcibly changing the air-fuel ratio of a predetermined cylinder and calculating the first parameter;
(D) calculating a difference between the first parameters before and after the forced change;
(E) determining a first characteristic representing a relationship between the second parameter and the difference based on a possible range of the second parameter and the difference;
(F) correcting one of the determination value and the first parameter calculated in the step (A) based on the determined slope of the first characteristic;
The inter-cylinder air-fuel ratio variation abnormality detecting device is configured to execute In step (E),
Using the first characteristic predetermined for the tolerance upper limit product and the tolerance lower limit product of the air-fuel ratio sensor, the difference calculated in the step (D) and the range that the second parameter can take The first characteristic passing through the intersection with a predetermined value is obtained by interpolation from the first characteristic of the tolerance upper limit product and the tolerance lower limit product, and the obtained first characteristic is determined as the first characteristic to be determined. The inter-cylinder air-fuel ratio variation abnormality detecting device according to claim 1. 3. The inter-cylinder space according to claim 2, wherein a predetermined value in a range that can be taken by the second parameter is a value that has a minimum degree of variation in a range that can be taken by the second parameter. Fuel ratio variation abnormality detection device. The first characteristic that is predetermined for the tolerance upper limit product and the tolerance lower limit product of the air-fuel ratio sensor is such that the difference increases as the second parameter changes to the larger variation degree side. The inter-cylinder air-fuel ratio variation abnormality detecting device according to claim 2 or 3. 5. The inclination of the first characteristic predetermined for the tolerance upper limit product is larger than the inclination of the first characteristic predetermined for the tolerance lower limit product. 6. The inter-cylinder air-fuel ratio variation abnormality detection device described. In step (B),
The first parameter calculated in the step (A) using a second characteristic representing a relationship between the first and second parameters, which is predetermined for a tolerance upper limit product and a tolerance lower limit product of the air-fuel ratio sensor. The range of the second parameter between the intersections of the upper limit tolerance product and the tolerance lower limit product with the second characteristic is determined as a possible range of the second parameter. The inter-cylinder air-fuel ratio variation abnormality detection device according to claim 1. The cylinder according to any one of claims 1 to 6, wherein the steps (A) and (C) include a step of normalizing the first parameter in accordance with an operating state of the internal combustion engine. Inter-air-fuel ratio variation abnormality detection device.

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